Hydraulic systems in vehicular applications, such as transmissions or braking systems, must engage controllably, responsively and reliably in order to perform their function. The same is true for hydraulically actuated clutches, or other power transmitting apparatus, used in stationary emergency power generators which supply electricity to a computer or a hospital for instance, in the event of a power outage in a municipal power grid.
A typical hydraulic system in these applications includes a pump or other source of pressurized fluid, a low pressure reservoir, a source of control signals, a fluid actuated device, and a fluid control valve. These components are connected to form a fluid circuit in which the valve receives a flow of pressurized fluid from the pump. The valve converts a portion of that flow of pressurized fluid into control flow or pressure that is then supplied to the fluid actuated device in response to control signals received from the source or control signals. The remaining small leakage portion of the flow of pressurized fluid passes through the valve and returns to the low pressure reservoir.
Such hydraulic systems must be capable of responding quickly when they are needed, despite the fact that the hydraulic fluid may have become highly viscous during a prolonged period of inoperation coupled with exposure to extremely cold ambient temperatures. Such highly viscous fluid does not flow readily, however, thus making it difficult to achieve rapid response. Furthermore, once the system has been actuated, the fluid will begin to warm up and become less viscous, due to frictional and dynamic losses in the fluid circuit. This change in viscosity can create significant difficulties in maintaining stable control. Therefore, in order to maintain stable control, the components of the hydraulic system should ideally include compensation features which allow the system to operate satisfactorily over a wide range of fluid temperatures and viscosity.
In addition, hydraulic control systems in both vehicular or stationary applications are subject to contamination. Such contaminations can cause control elements within the fluid circuit to jam, or stick, thereby preventing the valve from functioning properly.
In the past, designers of hydraulic systems have been compelled, by lack of acceptable alternatives, to utilize high fluid pressures, two-stage valves, or fluid heating/cooling devices in systems such as those described above. The heating/cooling devices were utilized to maintain fluid temperature and viscosity within a narrow range to alleviate control problems incident with changing fluid viscosity, as discussed above. High fluid pressures are undesirable because they require excessive pumping power, thereby increasing operational cost. Two-stage valves and heating/cooling devices add undesirable complexity and initial cost of the system. Heating/cooling devices which require external power may also increase operational costs. In other prior hydraulic systems without dedicated heating and cooling devices, it was sometimes necessary to run the pump for some period of time prior to attempting to engage the fluid actuated device, in order to let the fluid warm up to a temperature at which the valve would provide stable, responsive control. This warm up period was a waste of fuel. Furthermore, the time required for warm up sometimes resulted in unacceptable delays in the operational readiness of the hydraulic system.
The fluid actuated device, in a typical hydraulic system of the type described above, is a clutch or brake utilized to controllably start or stop a mechanical load, such as drivetrain or a wheel. In general, drivetrains and wheels are mechanical loads having components of inertia, variable torque, and viscous drag. The inertial load results in a system that has an integral time relationship between applied torque and resulting speed. If a simple flow control valve is utilized, for instance, a double integration occurs in the overall control algorithm, making it difficult to achieve stability. Specifically, this double integration occurs because the relationship between the flow control valve and the clutch creates a second time integral relationship between the control valve input and the resulting torque applied by the clutch. The combined effect of these two time integral relationships leads to difficulties in maintaining system stability. If, on the other hand, a pressure control valve with feedback of control pressure is used, the valve/clutch characteristic is proportional rather than integral in nature. No double integration occurs, and stability is thus much easier to achieve.
What is needed then is a hydraulic system including a low pressure, single stage, pressure control valve with internal feedback of control pressure. The valve must be capable of operating over a wide range of viscosities without the need for heating/cooling devices in the fluid circuit. The valve must also work reliably despite the presence of contamination within the fluid. It is also highly desirable that the hydraulic control system provide a constant relationship between the magnitudes of the control signal input and the acceleration rate of the drive mechanism.
Previously known pressure servo control valves fall short of meeting these requirements. Generally, the previously known pressure servo control valves are of the two stage hydraulic amplified type. These valves use closely fitting movable spools, or small flapper/orifice type valves. These valves are more prone to failure due to contamination, and slow response with higher viscosity fluids.
U.S. Pat. No. 3,805,835 to Harvey B. Jansen, a co-inventor of the present invention, describes a fluid flow control valve which utilizes a bifurcated clevis member that is shiftable across a pair of opposed metering orifices to precisely control flow of a fluid. By virtue of its construction, the Jansen '835 flow control valve is more tolerant of contaminated fluids than other types of flow control valves. The bifurcated clevis tends to scrape away contaminant deposits which could plug metering orifices or cause sticking of movable metering elements in other types of flow control valves. The structure of the Jansen '835 valve also minimizes mechanical hysteresis of the valve, and requires less power for actuation than other types of flow control valves. Frequency response of the Jansen '835 valve is also excellent, even at high operating frequencies.
The Jansen '835 valve is a flow control valve, however, rather than a pressure control valve of the type required to solve the problems addressed by the present invention. Despite the fact that the Jansen '835 valve is a flow control, the inventors of the present invention recognized that portions of the structure of that valve might be well suited for use with the viscous and possibly contaminated fluids encountered in hydraulic control systems of the type addressed herein, provided that those desirable features of the Jansen '835 flow control valve could be incorporated into a remotely controllable pressure control valve. In addition to integrating the desirable features of the Jansen '835 flow control valve into a pressure control valve, a feedback of control pressure is desired for achieving optimal responsiveness and stability of the hydraulic system.